Redox potentials and kinetics of the Ce3+/Ce4+ redox reaction and solubility of cerium sulfates in sulfuric acid solutions

Experimental work was performed with the aim of evaluating the Ce4+/Ce3+ redox couple in sulfuric acid electrolyte for use in redox flow battery (RFB) technology. The solubility of cerium sulfates in 0.1–4.0 M sulfuric acid at 20–60 °C was studied. A synergistic effect of both sulfuric acid concentration and temperature on the solubility of cerous sulfate was observed. The solubility of cerous sulfate significantly decreased with rising concentration of sulfuric acid and rising temperature, while the solubility of ceric sulfate goes through a significant maximum at 40 °C. Redox potentials and the kinetics of the cerous/ceric redox reaction were also studied under the same temperature–concentration conditions. The redox potentials were measured using the combined redox electrode (Pt–Ag/AgCl) in equimolar Ce4+/Ce3+ solutions (i.e.[Ce3+]=[Ce4+]) in sulfuric acid electrolyte. The Ce3+/Ce4+ redox potentials significantly decrease (i.e. shift to more negative values) with rising sulfuric acid concentration; a small maximum is observed at 40 °C. Cyclic voltammetric experiments confirmed slow electrochemical kinetics of the Ce3+/Ce4+ redox reaction on carbon glassy electrodes (CGEs) in sulfuric acid solutions. The observed dependencies of solubilities, the redox potentials and the kinetics of Ce3+/Ce4+ redox reaction on sulfuric acid concentration are thought to be the result of inequivalent complexation of the two redox species by sulfate anions: the ceric ion is much more strongly bound to sulfate than is the cerous ion. The best temperature–concentration conditions for the RFB electrolytes appear to be 40 °C and 1 M sulfuric acid, where the relatively good solubility of both cerium species, the maximum of redox potentials, and the more or less satisfying stability of CGE s were found. Even so, the relatively low solubility of cerium salts in sulfuric acid media and slow redox kinetics of the Ce3+/Ce4+ redox reaction at carbon indicate that the Ce3+/Ce4+ may not be well suited for use in RFB technology.